This invention relates generally to detector circuits and, more particularly, to a zener referenced detector circuit which provides a positive hysteresis effect on the output current resulting in a defined trip point and noise immunity.
Circuits which generate an output when an input signal reaches a predetermined voltage or trip point are well known. It is also well known that if the input voltage is at or very near the trip point, unwanted voltage excursions such as those produced by noise will cause the detector circuit to switch on and off resulting in an undesirable and spurious output.
A detector circuit is known which comprises a PNP dual collector transistor (or first and second single collector PNP transistors coupled so as to operate as a dual collector transistor) having an emitter coupled to the input voltage via a resistor and having a first collector coupled to a load for supplying a current thereto. The base of the transistor is coupled via a zener diode to ground. A second collector of the transistor is coupled to the cathode of the zener diode so as to bias the zener in its breakdown region when current is flowing in the first and second collectors. Thus, when the input voltage reaches a predetermined trip point corresponding to the voltage of the zener diode plus the base-emitter voltage of the transistor (V.sub.BE) currents will begin to flow in the first and second collectors. These currents will be equal if the collector areas are equal or may be scaled to any desired ratio by properly scaling the collector areas. Unfortunately, if the input voltage is at or near the trip point, unwanted voltage excursions such as noise, will cause the transistor to turn on and off. This may have an adverse effect on the load which is coupled to the first collector of the transistor.